The process of film formation is fairly well understood in the coatings industry and details on the process may be found in "Organic Film Formers" in Paint and Surface Coatings, by J. Bentley (R. Lambourne (Editor), John Wiley and Sons, New York, N.Y., 1987). In particular, in film formation technology, it is known that the addition of a cross-linker to a film forming coating formulation can improve certain important properties of the formed coating, such as its hardness, solvent resistance and mar resistance. However, it is desirable to have a crosslinking mechanism which is operative during or after film formation. If the cross-linking mechanism has proceeded materially before film formation has been substantially completed, the overall film formation process will be compromised and the resultant film will be weak and porous with the result that the protective functions of the film will be severely impaired. In some applications heat may be used to assist crosslinking after film formation is complete. However, in many cases, heat is not available so the crosslinking mechanism is desirably triggered under ambient conditions before film formation is complete.
An example of a known good performance cross-linker for coating formulations is a product sold under the trade name of XAMA-7 (supplied by EIT Inc). XAMA-7 essentially consists of three connected aziridine groups. While XAMA-7 displays good cross-linking performance it does suffer from an important draw back. It is very toxic. It therefore has to be handled with the utmost of care. This is clearly undesirable.
However, not all commercial cross-linkers are as toxic as the aziridine based products. For example, carbodiimides having the general structural formula of r-N.dbd.C.dbd.N-r', wherein at least r' is an aliphatic group, have been used as cross-linkers. In this regard, U.S. Pat. No. 4,977,219, EP-A-0,277,361 and EP-A-0,241,805 disclose aliphatic polycarbodiimide cross-linkers, wherein both r and r' are aliphatic groups. EP-A-0277361 discloses a mixed aliphatic and aromatic polycarbodiimide cross-linker, wherein r is an aromatic group and r' is an aliphatic group. Even though these aliphatic polycarbodiimide cross-linkers are used commercially, they do suffer from certain draw backs. In particular, crosslinking is too facile with the result that the cross-linking mechanism is substantially completed before film formation is completed. Therefore, despite the fact that a rapid cross-linking step will generally lead to an increase in crosslink density and possibly to improvements in solvent resistance, it detracts from the process of film formation. For example, there is a decrease in the important property of mar resistance. The rapid cross-linking therefore leads to the formation of generally poor and brittle coatings as they are not well-knitted.
The present invention seeks to overcome the problems associated with the prior art cross-linkers. In particular, the present invention seeks to provide a relatively non-toxic cross-linker having a good performance in coatings, in particular aqueous coatings.
According to a first aspect of the present invention there is provided a method of crosslinking a coating binder polymer bearing at least two carboxylic acid groups by admixing an aromatic polycarbodiimide which contains only aromatic carbodiimide groups.
According to a second aspect of the present invention there is provided an aromatic polycarbodiimide having the general formula of either EQU E--O--{--Z--O}.sub.m --G--C(O)N(A)--[R--N.dbd.C.dbd.N--].sub.n --R'--N(A)C(O)--G--{O--Z'--}.sub.p --O--E'
or EQU E--O--{--Z--O--}.sub.m --G--C(O)N(A)--[(R--N.dbd.C.dbd.N--).sub.q --R'--N(A)C(0)--B--C(O)N(A)--].sub.t --R"--N(A)C(O)--G--{O--Z'--}.sub.p --O--E'
wherein m is an integer of from 1 to 15, preferably 1 to 10, more preferably 5 to 10, most preferably 7; n is an integer of from 2 to 15, preferably 2 to 10, more preferably 2 to 7, most preferably 7; p is an integer of from 1 to 15, preferably 1 to 10, more preferably 5 to 10, most preferably 7; q is an integer of from 1 to 15, preferably 1 to 10, more preferably 1 to 5, most preferably 3; t is an integer of from 1 to 15, preferably 1 to 10, more preferably 1 to 5, most preferably 3; A is independently selected from hydrogen or C.sub.1 -C.sub.6 alkyl, preferably all being hydrogen; B is a suitable spacer group which does not contain carbodiimide groups, preferably being an ether or ester spacer group; E and E' are hydrogen or an alkyl group such as a C.sub.1-10 alkyl group, preferably CH.sub.3, and may be the same or different; G is an optional spacer group which does not contain a carbodiimide group, such as g'--N(g)-- wherein g is selected from hydrogen and an alkyl group and g' is selected from a bond and a C.sub.1 -C.sub.6 alkyl group; R, R' and R" are independently selected from arylene, substituted arylene, biarylene alkylene and substituted biarylene alkylene; and Z is an alkyl group such as a C.sub.1-6 alkyl group, preferably CH.sub.2 CH.sub.2, and may be the same or different.
According to a third aspect of the present invention there is provided an aqueous dispersion of an aromatic carbodiimide of the second aspect of the invention.
According to a fourth aspect of the present invention there is provided a water dispersable, polyether-terminated aromatic polycarbodiimide.
According to a fifth aspect of the present invention there is provided a process for preparing an aromatic polycarbodiimide as defined in any one of the preceding claims comprising reacting an aromatic isocyanate, preferably a polyisocyanate more preferably an aromatic diisocyanate, which may be the same or different, with a suitable reactant to form the aromatic polycarbodiimide.
An advantage of the present invention is that the aromatic polycarbodiimides are relatively non-toxic.
A further advantage is that the cross-linking process is much slower than that of the prior art aliphatic carbodiimide systems. It is believed that the crosslinking process may be approximately fifty times slower. The cross-linking process is slow enough for substantially all of it to occur during or after film formation with the result that a number of important requirements for a good coating, such as mar resistance, are greatly improved. The crosslinking is also fast enough to be useful in an ambiently cured film in a typical industrial or architectural use.
Further advantages of the present invention are that the water solubility of the aromatic polycarbodiimides can be tailored to suit individual needs, for example by incorporating surfactant groups. Also, the compatibilty of the aromatic polycarbodiimides with specific polymer (or resin) systems can be tailored to suit individual needs, for example by incorporating suitable compatibilising groups. For example, pendent or terminal phenyl groups make the aromatic polycarbodiimides suitable for use with polyester based polymers; whereas pendant or terminal polyether groups make the aromatic polycarbodiimides suitable for use with acrylic polymers.
The present invention therefore rests in the discovery and recognition of a new use of aromatic polycarbodiimides as well as in the preparation of new aromatic polycarbodiimides. In this regard, even though certain aromatic polycarbodiimides (i.e. wherein both r and r' of r--N.dbd.C.dbd.N--r' are aromatic groups) are known from publications such as U.S. Pat. No. 5,126,422, EP-B-0,231,509, U.S. Pat. No. 3,450,562, and U.S. Pat. No. 2,941,983, none of them falls within the scope of the above claims. Moreover, none of those documents specifically suggests or reports the use of aromatic polycarbodiimides as cross-linkers. The same is true for U.S. Pat. No. 4,612,054 which discloses carbodiimide driers for resin coating compositions. Whilst the general formula presented for the polycarbodiimide could cover an aromatic polycarbodiimide, there is no specific mention of, let alone a technical teaching for, both the use of the aromatic polycarbodiimides as cross-linkers and an aromatic polycarbodiimide according to the present invention.
The aromatic polycarbodiimide according to the present invention is water-soluble or readily emulsifiable in the presence of water. In particular, the aromatic polycarbodiimide is very effective as a cross-linking agent for carboxyl-containing, water borne polymers or resins (e.g. emulsion-polymerized acrylic polymer). The carboxyl-containing polymers may be totally or partially neutralised with a suitable base. The polycarbodiimide is also an effective crosslinking agent for other polymers based on epoxides, polyesters, and polyurethanes. In theory, the aromatic polycabodiimides according to the present invention may be used as a cross-linker in any water-borne or solvent-borne thermoset coating application.
Preferably, in the method of this invention, the aromatic polycarbodiimide comprises at least one pendant or terminal group for compatibilising the aromatic polycarbodiimide with a polymer in or as the coating formulation, preferably wherein the polymer is an acrylic polymer and preferably wherein the compatibilising group is an ether group.
Optionally, in the method of this invention, the aromatic polycarbodiimide comprises at least two aromatic carbodiimide groups separated by a spacer group, preferably the spacer group comprises at least one ether or ester group.
Preferably, in the method of this invention, the aromatic polycarbodiimide includes at least one substituted arylene group, preferably being derived from an aromatic polyisocyanate.
Preferably, in the method of this invention, the aromatic polycarbodiimide has from 4 to 10 aromatic carbodiimide groups, preferably about 5 to 7 groups. More preferably, the aromatic polycarbodiimide has about 7 carbodiimide groups.
Preferably, in the method of this invention, the aromatic polycarbodiimide has the general formula EQU X--[R--N.dbd.C.dbd.N].sub.n --R'--Y
wherein X is a terminal group; Y is an optional terminal group; R and R' are independently selected from arylene, substituted arylene, biarylene alkylene and substituted biarylene alkylene; and n is an integer greater than 1.
Preferably, in the method of this invention, each of R and R' is derived from an aromatic polyisocyanate, which may be the same or different, preferably an aromatic diisocyanate such as toluene diisocyanate, naphthalene diisocyanate and diphenylmethane 4,4'-diisocyanate, most preferred is toluene diisocyanate.
Preferably, X or Y is a compatibilising group for an acrylic polymer. Preferably both X and Y are groups containing ether linkages which may be the same or different. Preferably, X and Y are similar groups containing ether linkages.
Optionally, in the use, the aromatic polycarbodiimide has the general formula as defined above for the second aspect according to the present invention. Preferably, q.times.t, the product of q and t, is from 4 to 10, preferably from 6 to 9, more preferably 9.
Preferably, in the above general formulae for the aromatic polycarbodiimide, m is about 7 and/or n is about 7 and/or p is about 7 and/or q is about 3 and/or t is about 3 and/or A is hydrogen and/or R, R' and R" are independently selected from arylene, substituted arylene, biarylene alkylene and substituted biarylene alkylene and/or Z is CH.sub.2 CH.sub.2 and/or q.times.t is about 9. Preferably, each of R, R' and R" is the residue of an aromatic polyisocyanate, which may be the same or different, preferably an aromatic diisocyanate. B may be an ether group such as --O--(--CH.sub.2 CH.sub.2 O).sub.1-6 -- or an ethoxy group with pendant ester groups such as --O--CH{C(O)OCH.sub.3 }--CH{C(O)OCH.sub.3 }--O--.
Preferably, the aromatic polycarbodiimide has the following formula EQU CH.sub.3 O-{--Z--O}.sub.m --C(O)N(A)--[R--N.dbd.C.dbd.N--].sub.n --R'--N(A)C(O)--{O--Z-}.sub.p --OCH.sub.3
wherein m is 7; n is 7; p is 7; A is hydrogen; R and R' are substituted arylene; and Z is CH.sub.2 CH.sub.2.
The aromatic polycarbodiimide may therefore have the following formula ##STR1##
Another aromatic polycarbodiimide according to the present invention has the following formula ##STR2##
Preferably, the aromatic polycarbodiimides according to the present invention are prepared by polymerising aromatic mono-, di-, or tri-functional isocyanates.
Aromatic polycarbodiimides prepared by the condensation polymerization of diisocyanate with phospholene oxide catalyst are suseptible to thickening over the course of time. This thickening makes it increasingly difficult to prepare formulations containing the polycarbodiimide and eventually renders the product unusable for some purposes. It has been found that including certain stabilizers, in an amount effective to reduce the tendency of an aromatic polycarbodiimides to thicken, in the preparation of aromatic polycarbodiimides, extends their useful lifetime. Examples of such stabilizers include hindered phenols such as tetrakis(methylene 3-(3', 5'-di-tert-butyl-4'-hydroxyphenyl)proponate) methane (marketed as IRGANOX 1010; IRGANOX is a trademark of Ciba-Geigy Corp.) and 2,4-dimethyl-6-t-butyl phenol (marketed as AO-30 by Ciba-Geigy Corp.); and hindered nitroxyls such as 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy, or mixtures thereof.
The coating formulation may be free of organic solvent. Or the coating formulation may contain a coalescing solvent.
The coating formulation may contain typical coating additives such as binders, fillers, defoamers, other cross-linkers, catalysts, surfactants, stabilisers, anti-flocculants, pigments and suitable solvents, such as water-miscible solvents or even water. The coating may also contain typical additives that are used for the specific end purpose, such as a tackifier in adhesives.
The coating binder polymer (or resin) can be selected according to the criteria generally skilled in the art of coating compositions. Preferably, the coating comprises a polymer (or resin) prepared from at least one of the following monomers: an ethylenically-unsaturated monocarboxylic acid such as (meth)acrylic acid; an (meth)acrylic ester monomer including long chain (meth)acrylates, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, secondary butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, neopentyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, dodecyl pentadecyl (meth)acrylate, cetyl-eicosyl (meth)acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate, bornyl (meth)acrylate, isobornyl (meth)acrylate, myristyl (meth)acrylate, pentadecyl (meth)acrylate, stearyl (meth)acrylate, and the like; hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate; (meth)acrylamide or substituted (meth)acrylamides; styrene or substituted styrenes; butadiene; vinyl acetate or other vinyl ester; (meth)acrylonitrile; a multi-ethylenically unsaturated monomer such as allyl (meth)acrylate, diallyl phthalate, 1,4-butylene glycol di(meth)acrylate, 1,6-hexanediol- di(meth)acrylate, trimethylolpropane tri(methyl)acrylate; an ethylenically-unsaturated dicarboxylic acid or half ester thereof or the anhydride thereof such as itaconic acid, fumaric acid, maleic acid, monomethyl itaconate, monomethyl fumarate, monobutyl fumarate, maleic anhydride; butylaminoethyl (meth)acrylate, di(methyl)aminoethyl (meth)acrylate; a monomer containing a a, b-unsaturated carbonyl functional groups such as fumarate, maleate, cinnamate and crotonate; or any other nonmentioned hydrophobic or hydrophilic polymerisable monomer.
The coating binder polymer contains at least two carboxylic acid groups. Preferably, the polymer has an acid number of from 5 to 100, preferably from 10 to 85, more preferably from 15 to 45, more preferably from 20 to 40.
Most preferably, the binder polymer is a stryene acrylic polymer, preferably with an acid number of from 10 to 85, more preferably from 15 to 45, more preferably from 20 to 40.
Preferably, the aromatic polycarbodiimide is a cross-linker for wood coatings, especially water-borne wood coatings. However, the aromatic polycarbodiimide can also be a cross-linker for maintenance coatings, metal primers and coatings, woven and nonwoven textile coatings, leather coatings, coil coatings, architectural coatings, mastics, sealants, caulks, board coatings, paper coatings, plastics coatings and adhesives.
The present invention therefore provides an aromatic polycabodiimide cross-linker which cross-links a coating formulation during or after film formation and so does not materially impair the quality of film formation.